Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped x-ray beam toward a subject or object, such as a patient or a piece of luggage, positioned on a support. The beam, after being attenuated by the subject, impinges upon a detector assembly. The intensity of the attenuated x-ray beam received at the detector assembly is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector assembly produces a separate electrical signal indicative of the attenuated x-ray beam received.
In known third generation CT systems, the x-ray source and the detector assembly are rotated on a rotatable gantry portion around the object to be imaged so that a gantry angle at which the fan-shaped x-ray beam intersects the object constantly changes. Data representing the strength of the received x-ray beam at each of the detector elements is collected across a range of gantry angles. The data are ultimately processed to form an image of the object.
It is commonly known that a percentage of the photons in the x-ray beam will interact with the subject or the support and that the interaction will result in the generation of secondary x-ray photons. Collectively, these secondary x-ray photons are commonly referred to as scattered x-ray photons. It is well-known that scattered x-ray photons travel in directions that are not predictable and may be a source of noise in the image. Since the x-ray photons are a source of noise, they need to be effectively managed before reaching the detector elements in order to maintain an acceptable signal-to-noise-ratio.
In known third generation CT scanners, the effects of scattered x-ray photons are managed through a post patient collimator. The post patient collimator is mounted on the detector assembly and it is located in a position between the subject and the detector elements. Typically the post patient collimator includes a plurality of thin collimation plates comprising a highly x-ray attenuating material such as tungsten, lead, molybdenum, etc. that are focally aligned to a focal spot of the x-ray source. Most scattered x-ray photons travel along paths that are not focally aligned to the focal spot of the x-ray source and, therefore, the scattered x-ray photons are effectively blocked by the highly x-ray attenuating material in the collimation plates. The post patient collimator is also used to control off-focus x-ray photons. For the purposes of this disclosure, off-focus x-ray photons are defined to include x-ray photons that originate from the x-ray source, but do not pass through the focal spot. The post patient collimator is designed to block both off-focus x-ray photons and scattered x-ray photons.
In order for the post patient collimator to perform acceptably well, it is critical that the thin collimation plates remain accurately aligned to the focal spot of the x-ray source. If the collimation plates are not accurately aligned, they may allow some of the off-focus x-ray photons or the scattered x-ray photons to impinge upon the detector elements. It is well-known that either off-focus x-ray photons or scattered x-ray photons contacting the detector elements will result in a lower signal-to-noise ratio, which causes a degradation in image quality.
For known third generation CT systems, a fast gantry rotation speed is necessary to obtain high temporal resolution in the image. However, as the gantry rotation speed increases, the collimation plates are subjected to a significantly larger deflecting force. Since the rotatable gantry portion is rotating, the deflecting force acting on each collimation plate increases as the square of the gantry rotation speed. Because of this relationship, even a small increase in the gantry rotation speed leads to a significantly higher force acting to deflect each of the collimation plates. The problem is that faster gantry rotation speeds can lead to significant deflection of the collimation plates, allowing more off-focus x-ray photons and scattered photons to contact the detector elements, which ultimately causes a degradation in image quality.
A known solution is to brace the collimation plates with tungsten wires in an x-direction of the detector assembly. Since tungsten is highly x-ray attenuating, the tungsten wires absorb some of the x-ray photons that have not been scattered before they reach the detector elements. Using tungsten wires to brace the collimation plates may not be desirable since the tungsten wires blocks x-ray photons that would otherwise be used to create a higher quality image for a given x-ray dose. In addition to absorbing x-ray photons, the tungsten wires may also demark where x-rays may pass, which can induce image artifacts.